This invention relates to an arrangement for digital multiplexing onto a common highway of a plurality of analogue input signals, particularly where the analogue inputs are physically spaced apart over a distance.
Typical examples of the application of the invention are a hydrophone listening system where the acoustic signals from a large number of hydrophone transducers are required to be transmitted to a single control and data processing unit. Some of the problems encountered in such a system are as follows:
(i) Phase delay. When a number of hydrophones are distributed linearly over a distance, say 100 meters, there will be absolute delay difference between the arrivals of analogue signals at a common point, e.g. a multiplexer at one end of the system, and differential frequency delay will occur across the band of frequencies in the spectrum of the signals. Both these delays will cause a differential distortion of the hydrophone signals.
(ii) Cross talk. With a number of signals travelling in channels in the same cable a level of cross talk between those channels will exist. This cross talk cannot be removed by filtering because it will be `in-band`.
(iii) Attenuation. The attenuation which will exist between the hydrophone and a multiplexer will be different for each channel and therefore a measure of adjustment or calibration will be necessary.
(iv) Filtering. When filtering is applied on a one filter per analogue channel basis before multiplexing it must be very accurate and necessitates costly filter units so that no added distortion is introduced in a different degree into the individual channels.
These problems can be avoided in an all digital approach, that is one where the individual acoustic signals are digitally encoded as near the source as possible for onward transmission and multiplexing. Considering now the problems listed above the following advantages of the digital approach are:
(i) Phase delay. Delays in the analogue signals applied to a common encoder do not now exist. Any signal delays in the digital signals can be retimed or balanced out and very accurate phase relationships will exist between all the digital channel signals.
(ii) Cross talk. The cross talk between channels will now be between digital signals and can be eliminated by threshold detection or strobing.
(iii) Attenuation. Attenuation of the digital signals will not represent any attenuation of the analogue signal.
(iv) Filtering. The filtering can be carried out digitally according to a fixed law, this law being the same for each channel, and therefore no differential distortion will be introduced. Furthermore the digital filter can be multiplexed to perform the filter function on more than one channel, thereby reducing the cost of filtering.